U.S. patent application number 10/295032 was filed with the patent office on 2003-07-24 for steel for machine structural use.
Invention is credited to Kato, Toru, Matsui, Naoki, Matsumoto, Hitoshi, Nishi, Takayuki, Tahira, Hiroaki, Watari, Koji.
Application Number | 20030138343 10/295032 |
Document ID | / |
Family ID | 19162966 |
Filed Date | 2003-07-24 |
United States Patent
Application |
20030138343 |
Kind Code |
A1 |
Matsui, Naoki ; et
al. |
July 24, 2003 |
Steel for machine structural use
Abstract
The invention provides a steel for machine structural use, which
is excellent in machinability, comprising, in percent by mass, C:
0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, S: 0.005-0.2%, Al: not more
than 0.009%, Ti: not less than 0.001% but less than 0.04%, Ca:
0.0001-0.01%, O (oxygen): 0.001-0.01%, and N: not more than 0.02%
and satisfying the following relations (1) to (3): n.sub.0/S
(%).gtoreq.2500 (1) n.sub.1/n.sub.0.ltoreq.0.1 (2)
n.sub.2.gtoreq.10 (3) where n.sub.0: total number of sulfide
inclusions not smaller than 1 .mu.m per mm.sup.2 of a cross section
parallel to the direction of rolling (number/mm.sup.2); n.sub.1:
number of MnS inclusions having not smaller than 1 .mu.m and
containing not less than 1.0% of Ca per mm.sup.2 of a cross section
parallel to the direction of rolling (number/mm.sup.2); n.sub.2:
number, per mm.sup.2 of a cross section parallel to the direction
of rolling, of oxide inclusions having a specific composition
comprising CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.2 and having a
diameter of not less than 1 .mu.m (number/mm.sup.2).
Inventors: |
Matsui, Naoki;
(Amagasaki-shi, JP) ; Watari, Koji; (Kobe-shi,
JP) ; Nishi, Takayuki; (Kashima-shi, JP) ;
Kato, Toru; (Kashima-shi, JP) ; Matsumoto,
Hitoshi; (Kitakyushu-shi, JP) ; Tahira, Hiroaki;
(Amagasaki-shi, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN & HATTORI, LLP
1725 K STREET, NW
SUITE 1000
WASHINGTON
DC
20006
US
|
Family ID: |
19162966 |
Appl. No.: |
10/295032 |
Filed: |
November 15, 2002 |
Current U.S.
Class: |
420/126 |
Current CPC
Class: |
C22C 38/02 20130101;
C21C 7/06 20130101; C22C 38/002 20130101; C22C 38/60 20130101; C22C
38/04 20130101 |
Class at
Publication: |
420/126 |
International
Class: |
C22C 038/14 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 15, 2001 |
JP |
2001-350470 |
Claims
We claim:
1. A steel for machine structural use, consisting of, in percent by
mass, C: 0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, P: not more than
0.1%, S: 0.005-0.2%, Al: not more than 0.009%, Ti: not less than
0.001% but less than 0.04%, Ca: 0.0001-0.01%, O (oxygen):
0.001-0.01%, N: not more than 0.02% and the balance being Fe and
impurities, wherein the following relations (1) to (3) are
satisfied with respect to the inclusions in the steel:n.sub.0/S
(%).gtoreq.2500 (1),n.sub.1/n.sub.0.ltoreq.0.1
(2),n.sub.2.gtoreq.10 (3), wherein n.sub.0,n.sub.1 and n.sub.2 are
defined as follows: n.sub.0: total number of sulfide inclusions,
having a circle equivalent diameter of not less than 1 .mu.m, per
mm.sup.2 of a cross section parallel to the direction of rolling,
number/mm.sup.2; n.sub.1: number of MnS, having a circle equivalent
diameter of not less than 1 .mu.m and containing not less than 1.0%
of Ca, per mm.sup.2 of a cross section parallel to the direction of
rolling, number/mm.sup.2; n.sub.2: number, per mm.sup.2 of a cross
section parallel to the direction of rolling, of oxide inclusions
having a composition comprising
CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.2 and impurities, with
CaO: 5-60%, Al.sub.2O.sub.3: 5-60%, SiO.sub.2: 10-80% and
TiO.sub.2: 0.1-40% when the sum of CaO, Al.sub.2O.sub.3, SiO.sub.2
and TiO.sub.2 is taken as 100% by mass, and having a circle
equivalent diameter of not less than 1 .mu.m, number/mm.sup.2.
2. A steel for machine structural use consisting of, in percent by
mass, C: 0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, P: not more than
0.1%, S: 0.005-0.2%, Al: not more than 0.009%, Ti: not less than
0.001% but less than 0.04%, Ca: 0.0001-0.01%, O (oxygen):
0.001-0.01%, N: not more than 0.02%, and at least one element
selected from the first group consisting of Cr: 0.02-2.5%, V:
0.05-0.5%, Mo: 0.05-1.0%, Nb: 0.005-0.1%, Cu: 0.02-1.0% and Ni:
0.05-2.0%, and the balance being Fe and impurities, wherein the
following relations (1) to (3) are satisfied with respect to the
inclusions in the steel:n.sub.0/S (%).gtoreq.2500
(1),n.sub.1/n.sub.0.ltoreq.0.1 (2),n.sub.2.gtoreq.10 (3), wherein
n.sub.0, n.sub.1 and n.sub.2 are defined as follows: n.sub.0: total
number of sulfide inclusions, having a circle equivalent diameter
of not less than 1 .mu.m, per mm.sup.2 of a cross section parallel
to the direction of rolling, number/mm.sup.2; n.sub.2: number of
MnS, having a circle equivalent diameter of not less than 1 .mu.m
and containing not less than 1.0% of Ca, per mm.sup.2 of a cross
section parallel to the direction of rolling, number/mm.sup.2;
n.sub.2: number, per mm.sup.2 of a cross section parallel to the
direction of rolling, of oxide inclusions having a composition
comprising CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.- 2 and
impurities, with CaO: 5-60%, Al.sub.2O.sub.3: 5-60%, SiO.sub.2:
10-80% and TiO.sub.2: 0.1-40% when the sum of CaO, Al.sub.2O.sub.3,
SiO.sub.2 and TiO.sub.2 is taken as 100% by mass, and having a
circle equivalent diameter of not less than 1 .mu.m,
number/mm.sup.2.
3. A steel for machine structural use consisting of, in percent by
mass, C: 0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, P: not more than
0.1%, S: 0.005-0.2%, Al: not more than 0.009%, Ti: not less than
0.001% but less than 0.04%, Ca: 0.0001-0.01%, O (oxygen):
0.001-0.01%, N: not more than 0.02%, and at least one element
selected from the second group consisting of Se: 0.0005-0.01%, Te:
0.0005-0.01%, Bi: 0.05-0.3% and rare earth elements:
0.0001-0.0020%, and the balance being Fe and impurities, wherein
the following relations (1) to (3) are satisfied with respect to
the inclusions in the steeln.sub.0/S (%).gtoreq.2500
(1),n.sub.1/n.sub.0.ltoreq.0.1 (2),n.sub.2.gtoreq.10 (3), wherein
n.sub.0, n.sub.1 and n.sub.2 are defined as follows: n.sub.0: total
number of sulfide inclusions, having a circle equivalent diameter
of not less than 1 .mu.m, per mm.sup.2 of a cross section parallel
to the direction of rolling, number/mm.sup.2; n.sub.1: number of
MnS, having a circle equivalent diameter of not less than 1 .mu.m
and containing not less than 1.0% of Ca, per mm.sup.2 of a cross
section parallel to the direction of rolling, number/mm.sup.2;
n.sub.2: number, per mm.sup.2 of a cross section parallel to the
direction of rolling, of oxide inclusions having a composition
comprising CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.- 2 and
impurities, with CaO: 5-60%, Al.sub.2O.sub.3: 5-60%, SiO.sub.2:
10-80% and TiO.sub.2: 0.1-40% when the sum of CaO, Al.sub.2O.sub.3,
SiO.sub.2 and TiO.sub.2 is taken as 100% by mass, and having a
circle equivalent diameter of not less than 1 .mu.m,
number/mm.sup.2.
4. A steel for machine structural use consisting of, in percent by
mass, C: 0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, P: not more than
0.1%, S: 0.005-0.2%, Al: not more than 0.009%, Ti: not less than
0.001% but less than 0.04%, Ca: 0.0001-0.01%, O (oxygen):
0.001-0.01%, N: not more than 0.02%, at least one element selected
from the first group consisting of Cr: 0.02-2.5%, V: 0.05-0.5%, Mo:
0.05-1.0%, Nb: 0.005-0.1%, Cu: 0.02-1.0% and Ni: 0.05-2.0%, and at
least one element selected from the second group consisting of Se:
0.0005-0.01%, Te: 0.0005-0.01%, Bi: 0.05-0.3% and rare earth
elements: 0.0001-0.0020%, and the balance Fe and impurities,
wherein the following relations (1) to (3) are satisfied with
respect to the inclusions in the steel:n.sub.0/S (%).gtoreq.2500
(1),n.sub.1/n.sub.0.ltoreq.0.1 (2),n.sub.2.gtoreq.10 (3), wherein
n.sub.0, n.sub.1 and n.sub.2 are defined as follows: n.sub.0: total
number of sulfide inclusions, having a circle equivalent diameter
of not less than 1 .mu.m, per mm.sup.2 of a cross section parallel
to the direction of rolling, number/mm.sup.2; n.sub.1: number of
MnS, having a circle equivalent diameter of not less than 1 .mu.m
and containing not less than 1.0% of Ca, per mm.sup.2 of a cross
section parallel to the direction of rolling, number/mm.sup.2;
n.sub.2: number, per mm.sup.2 of a cross section parallel to the
direction of rolling, of oxide inclusions having a composition
comprising CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.- 2 and
impurities, with CaO: 5-60%, Al.sub.2O.sub.3: 5-60%, SiO.sub.2:
10-80% and TiO.sub.2: 0.1-40% when the sum of CaO, Al.sub.2O.sub.3,
SiO.sub.2 and TiO.sub.2 is taken as 100% by mass, and having a
circle equivalent diameter of not less than 1 .mu.m,
number/mm.sup.2.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a steel for machine structural
use, which is to be subjected to machining for use as industrial
machinery or automotive parts, among others. More particularly, the
invention relates to a steel for machine structural use excellent
in chip disposability and effective in prolonging the cutting tool
life (hereinafter referred to as "tool life improvement").
PRIOR ART
[0002] Among the steels for machine structural use, which are used
as industrial machinery or automotive parts, among others, there
are steels for machine structural use as defined in the Japanese
Industrial Standard JIS G 4051, and such alloy steels as
nickel-chromium steels according to JIS G 4102,
nickel-chromium-molybdenum steels according to JIS G 4103, chromium
steels according to JIS G 4104 and manganese and manganese-chromium
steels for machine structural use according to JIS G 4106. Also in
use are steels improved in hardenability by modifying the amount of
addition of the specified components of these steels or by adding B
(boron) or the like and/or improved in metallurgical structure by
addition of Ti, Nb, V and/or the like.
[0003] In many cases, these steels are subjected, after rolling or
after further forging or other working, to machining to desired
forms or shapes, followed by heat treatment according to the
required characteristics, to give final products. For improving the
productivity in this machining step, it is strongly desired that
the steels be excellent in machinability. Good machinability means
that the period between exchanges of tools for use in machining due
to wear is long, that is, that the tool life is long, that chips
generated during machining can be finely torn and separated, that
the cutting force is not so great, and that good machined or ground
surfaces can be obtained.
[0004] With the advancement in automation of machining, not only
the tool life but also the separability of chips, namely "chip
disposability", becomes very important. Since the tool life is
influenced by the characteristics of the material steel as well as
the performance characteristics of the tool, tool selection is also
important. On the contrary, good chip disposability means that
chips generated during machining are finely torn or divided and
separated but will not entwine the tool. The chip disposability
greatly depends on the characteristics of the material steel. For
improving the machinability of steel, it is very important to
improve this chip disposability.
[0005] The machinability of steel can be improved by addition of
Pb. However, addition of Pb not only increases the cost of steel
but also may lead to environmental contamination. Therefore,
investigations have been carried out in search of technologies of
improving the machinability of steel without adding Pb. A typical
one is the technology of improving the machinability by utilizing
MnS inclusions. This technology has been studied in various aspects
and already put to practical use.
[0006] Thus, for example, the steel disclosed in Japanese Patent
Publication (JP Kokoku) H05-15777 contains Mn--Ca--S type
inclusions with a Ca content of 3-55% as uniformly dispersed
therein. As for their sizes, the major axis L is not longer than 20
.mu.m and the ratio thereof to the minor axis W (L/W) is not more
than 3. In this steel, however, individual sulfide inclusions
become coarse, hence the number of sulfide inclusions at the same S
concentration decreases. Therefore, the improvement in chip
disposability is not entirely satisfactory. In addition, because
the steel is Al-killed steel, even after treatment with Ca, the
oxide inclusions are of the CaO--Al.sub.2O.sub.3 type, hence the
improving effects on the machinability such as tool life are not
very satisfactory. When an attempt is made to disperse a large
number of sulfide inclusions containing a high concentration of CaS
by increasing the S concentration, addition of a large amount of Ca
is required, and this disadvantageously causes an increase in
cost.
[0007] Laid-open Japanese Patent Application (JP Kokai) 2001-131684
discloses steels for machine structural use, in which manganese
sulfide-based inclusions have an average oxygen content of not more
than 10%. The steels have the following principal composition, in %
by mass: C: 0.05-0.7%, Si: not more than 2.5%, Mn: 0.1-3.0%, Al:
not more than 0.1%, S: 0.003-0.2%, N: 0.002-0.025%, and O (oxygen):
not more than 0.003%, with the balance being Fe. In addition to
these components, the steels may contain not more than 0.01%, in
total, of one or more elements selected from the group consisting
of rare earth elements, Ca and Mg.
[0008] However, the steels according to the invention disclosed in
JP Kokai 2001-131684, as described in the example section thereof,
contain not less than 0.018% of Al used as a deoxidizer element so
that the average oxygen concentration in sulfides may be reduced to
10% or less for obtaining such a sulfide form as effective in
improving the chip disposability. In such a case, the oxides in
steel are mainly hard Al.sub.2O.sub.3 type oxides, and the tool
life is improved only to an unsatisfactory extent. Thus, the
invention disclosed in the above-cited publication is not an
invention made in an attempt to improve the chip disposability and,
at the same time, improve the tool life.
[0009] In JP Kokai 2000-34538, there is disclosed a steel for
machine structural use which contains C, Si, Mn, P, S, Al, Ca and N
each in a specified amount and is excellent in machinability in
turning. This steel has the following characteristic features.
Namely, the following two relations are satisfied:
A/(A+B+C).ltoreq.0.3 and B/(A+B+C).gtoreq.0.1
[0010] wherein A is the area percentage of sulfide inclusions
having a Ca content exceeding 40% relative to the total area of an
investigation field of view, B is the area percentage of sulfide
inclusions having a Ca content of 0.3-40% relative to the total
area of the investigation field of view, and C is the area
percentage of sulfide inclusions having a Ca content of less than
0.3% relative to the total area of the investigation field of view.
The steel of JP Kokai 2000-34538 is characterized by increasing
sulfide containing 0.3-40% of Ca. However, increase of such sulfide
of high Ca content makes the sulfide coarse and makes improvement
of chip disposability difficult.
[0011] JP Kokai 2000-282169 discloses a steel, which contains C,
Si, Mn, P and S and further contains one or more elements selected
from among Zr, Te, Ca and Mg and satisfies the conditions:
Al.ltoreq.0.01%, total O.ltoreq.0.2% and total N.ltoreq.0.02%. This
steel is excellent in forgeability owing to spheroidizing of
sulfide inclusions and has good machinability. Thus, on the premise
that Ca is added, it is intended that Ca solutes in MnS and lowers
the deforming ability of MnS for spheroidizing the same in this
steel. In this case, however, individual sulfide inclusions become
coarse, whereby that sulfide morphology suited for providing good
chip disposability cannot be obtained, hence the improvement in
chip disposability is not yet satisfactory.
[0012] The all steels disclosed in the above mentioned publications
may contain Ca and are improved primarily in machinability.
However, it cannot be said that sufficient considerations have been
given to the level of addition of Ca, the timing of addition
thereof and the dissolved oxygen content in the steel. Thus, they
are not satisfactorily improved simultaneously in chip
disposability and in tool life.
[0013] It is an object of the present invention to provide a steel
for machine structural use, which is improved in machinability,
especially in chip disposability and, at the same time, can prolong
the tool life, without containing Pb.
SUMMARY OF THE INVENTION
[0014] It is well known that the machinability of steel is greatly
influenced by the state of sulfide and/or oxide inclusions in the
steel. For improving the machinability of Pb-free steels for
machine structural use, the present inventors made close
investigations concerning the relationship between the form and
distribution of inclusions in the steels and the machinability
thereof, and studied the investigation results. The inventors paid
attention to the effects of Ca and Ti, in particular, and
investigated the steelmaking conditions as well. In the process of
these investigations and studies, the inventors reveal the
following remarkable facts.
[0015] Ca strongly binds to S and alters the form of sulfide
inclusions, mainly MnS, and shows a large bonding strength with
oxygen, leading to stable oxide formation.
[0016] When Ca is added without paying any attention to the
steelmaking conditions, CaS or Ca-based oxides formed in the molten
steel serve as nuclei for the formation of MnS grains and the
number of sulfide inclusions having a Ca content of not less than
1% increases. It was found, however, that when, in is adding Ca,
the steelmaking conditions, such as the level of addition thereof,
the dissolved oxygen level and the timing of addition of Ca, are
appropriately selected, sulfide inclusions mainly composed of MnS
not containing Ca are formed in large amounts. Further, it was
revealed that the chip disposability of steel becomes improved only
in such case.
[0017] There are two type inclusions, i.e., sulfide type one and
oxide type one. Since minute inclusions such as precipitates are
not effective in machinability improvement, it was decided that the
size of inclusion should be evaluated in terms of the diameter of a
circle equivalent in area to the inclusion in the observation field
of view, and investigations were made regarding inclusions greater
in such diameter than a certain level.
[0018] As a result, it was found that when the number of almost
Ca-free sulfide exceeds 90%, or, in other words, when the number of
Ca-containing MnS type inclusions is less than 10%, particularly
good chip disposability can be obtained.
[0019] When compared at the same S content level, steels, in which
a large number of small sulfide inclusions are present, are
superior in chip disposability to steels in which a small number of
coarse sulfide inclusions are present. When an increased amount of
sulfides containing Ca as solid solution is present in the molten
steel or at the initial stage of solidification, they serve as
nuclei for crystallization of MnS, giving coarse sulfide
inclusions. Therefore, at the same S concentration, the number of
dispersed sulfide inclusions decreases and fine sulfide inclusions
are hardly formed. When, on the other hand, the amount of sulfide
inclusions containing Ca as solid solution is small, the sulfide
inclusions mostly form a large number of fine sulfide
inclusions.
[0020] A chip generated during machining is torn or separated when
stress is concentrated on inclusions in the deformed steel chip,
resulting in crack formation and propagation. Ca-free MnS type
inclusions tend to be deformed in the direction of working, for
example rolling, and many of them have an elongated form. When
large elongated inclusions are present, the anisotropy in
mechanical properties of a steel material increases and, in
addition, the number of inclusions to serve as points for stress
concentration and starting points of chipping decreases, hence no
good chip disposability can be obtained. On the other hand, when
there are a large number of small inclusions, the number of crack
starting points in chips subjected to deformation during machining
increases and, further, stress is concentrated on the inclusions
and crack propagation becomes readily promotable thereby. This is
presumably the cause of improvement in chip disposability.
[0021] The tool life is greatly influenced by the composition of
oxides contained in the steel. When Ca is added to convert oxides
to low-melting oxides, the tool life is markedly prolonged.
Therefore, treatment with Ca is essential. For attaining both the
above-mentioned sulfide control and oxide control simultaneously,
the steelmaking conditions before and after Ca treatment were
further examined in detail. As a result, the following facts were
revealed. It becomes possible to control the oxide inclusions so
that they may be composed of
CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.- 2 as main constituents
even within the same composition range, by restricting the contents
of those components showing a high level of interaction with oxygen
in steel, such as C, Si and Mn, causing S to be contained at a
specific level, reducing Al as far as possible, adding Ti and Ca
each at an appropriate addition level and at an appropriate time
and adjusting the level of dissolved oxygen. These oxide inclusions
are low in melting point and soft and are presumably effective not
only in tool life improvement owing to Ca and Ti contained therein
but also in producing starting points for cracking in chips and
promoting crack propagation.
[0022] The influences of the compositions with respect to C, Si, Mn
and so on and of the contents of Cr, Ni, Mo, B, Nb, V and other
elements, which are added for improving the strength,
hardenability, metallurgical structure and other properties of
steels for machine structural use, on the improvement in chip
disposability and tool life as attainable by such forms of sulfide
inclusions and oxide inclusions were examined. As a result, it
could be confirmed that while these elements may improve the
strength, hardenability and other mechanical characteristics of
steels, the effect of the invention, namely the improvement in
machinability with the same composition can be produced in the same
manner.
[0023] Accordingly, the present inventors further established the
limits to the chemical composition and to the states or forms of
inclusions and, as a result, have completed the present invention.
The gist of the invention is as follows.
[0024] (1) A steel for machine structural use consisting of, in
percent by mass, C: 0.1-0.6%, Si: 0.01-2.0%, Mn: 0.2-2.0%, P: not
more than 0.1%, S: 0.005-0.2%, Al: not more than 0.009%, Ti: not
less than 0.001% but less than 0.04%, Ca: 0.0001-0.01%, O (oxygen):
0.001-0.01%, and N: not more than 0.02%, and the balance Fe and
impurities, and satisfying the following relations (1) to (3) with
respect to the inclusions in the steel:
n.sub.0/S (%).gtoreq.2500 (1)
n.sub.1/n.sub.0.ltoreq.0.1 (2)
n.sub.2.gtoreq.10 (3)
[0025] wherein n.sub.0, n.sub.1 and n.sub.2 are defined as
follows:
[0026] n.sub.0: total number of sulfide, having a circle equivalent
diameter of not less than 1 .mu.m, per mm.sup.2 of a cross section
parallel to the direction of rolling, number/mm.sup.2;
[0027] n.sub.1: number of MnS, having a circle equivalent diameter
of not less than 1 .mu.m and containing not less than 1.0% of Ca,
per mm.sup.2 of a cross section parallel to the direction of
rolling, number/mm.sup.2;
[0028] n.sub.2: number, per mm.sup.2 of a cross section parallel to
the direction of rolling, of oxide inclusions having a composition
comprising CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.2 and
impurities, with CaO: 5-60%, Al.sub.2O.sub.3: 5-60%, SiO.sub.2:
10-80% and TiO.sub.2: 0.1-40% when the sum of CaO, Al.sub.2O.sub.3,
SiO.sub.2 and TiO.sub.2 is taken as 100% by mass, and having a
circle equivalent diameter of not less than 1 .mu.m,
number/mm.sup.2.
[0029] (2) A steel for machine structural use which comprises, in
addition to the components mentioned above in (1), one or more
elements selected from the first group and/or second group shown
below and satisfies the relations (1), (2) and (3) given above.
[0030] First group:
[0031] Cr: 0.02-2.5%, V: 0.05-0.5%, Mo: 0.05-1.0%, Nb: 0.005-0.1%,
Cu: 0.02-1.0% and Ni: 0.05-2.0%;
[0032] Second group:
[0033] Se: 0.0005-0.01%, Te: 0.0005-0.01%, Bi: 0.05-0.3% and rare
earth elements: 0.0001-0.0020%.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a graphic representation of the relationship
between chip disposability index and S content of steel.
[0035] FIG. 2 is a graphic representation of the relationship
between chip disposability index and "n.sub.1/n.sub.0" of
steel.
[0036] FIG. 3 is a graphic representation of the relationship
between chip disposability index and "n.sub.0/S (%)" of steel.
[0037] FIG. 4 is a graphic representation of the relationship
between tool life and S content of steel.
[0038] FIG. 5 is a graphic representation of the relationship
between tool life and n.sub.2 of steel.
DETAILED DESCRIPTION OF THE INVENTION
[0039] The grounds for the restrictions as to the inclusion
distribution, chemical composition and other aspects in or of the
steel of the invention are explained below. In the following
description "%" referring to steel constituents means "% by
mass".
[0040] The reason why only those inclusion, which have an
equivalent diameter of not smaller than 1 .mu.m as found upon
substituting a circle equivalent in area for each inclusion shape
observed on a cross section parallel to the direction of rolling,
are taken into consideration is that those inclusions smaller than
1 .mu.m have almost no effects on the tool life and chip
disposability. Those inclusions which have a diameter exceeding 10
.mu.m upon substitution with an equivalent circle impair the
strength and other steel characteristics, prevent inclusions from
being uniformly dispersed and are ineffective in improving the chip
disposability, in particular, hence are undesirable.
[0041] Many of the inclusions observed on a cross section parallel
to the direction of working show elongation in the direction of
working or are indefinite in shape. In shape examination, the cross
section of a steel sample is mirror-polished and photographed under
an optical microscope at a magnification of about 400, the area of
each inclusion is determined by a technique of image analysis and
the area thereof is converted to that of a circle, and those
inclusions having a diameter of not smaller than 1 .mu.m alone are
taken into consideration. On that occasion, when it can be judged
without doubt that two or more inclusions identical in composition
have been divided by rolling, they should be treated as one
inclusion. The composition of each inclusion is analyzed, for
example, using an EPMA (electron probe X-ray microanalyzer) or an
apparatus equivalent thereto and capable of analyzing microscopic
portions.
[0042] When the total number of MnS-containing inclusions, among
such inclusions as mentioned above, per mm.sup.2 is expressed as
n.sub.0 and the analytical value of S as S (%), the following
relation should be satisfied:
n.sub.0/S (%).gtoreq.2500 (1)
[0043] When the ratio n.sub.0/S (%) is below 2500, the number of
inclusions becomes smaller, the characteristics as a steel material
are poor and the chip disposability is also poor, when comparison
is made between steels having the same S content. The number of
inclusions decreases at the same S content level because individual
inclusions form coarser grains. Within a range in which the
relation (1) is satisfied, good chip disposability can be obtained.
For further stably obtaining good chip disposability, however, the
ratio n.sub.0/S (%) is desirably not less than 3500. So long as the
relation (1) is satisfied, no may be large. However, when no is
excessively large, it becomes difficult to obtain such mechanical
properties as tensile strength and fatigue strength as required of
steels for machine structural use. Therefore, it is preferred that
no be not more than 2000, more preferably not more than 1000.
[0044] When the number of those sulfide inclusions containing not
less than 1.0% by mass of Ca, among the sulfide inclusions, per
mm.sup.2 is expressed as n.sub.1, the following relation should be
satisfied:
n.sub.1/n.sub.0.ltoreq.0.1 (2)
[0045] This is because when the ratio of the number of sulfide,
containing not less than 1.0 mass % Ca, to the total number of
sulfide exceeds 0.1, there is a tendency toward individual
inclusions becoming coarser, leading to lowered chip disposability.
Within the range specified by the above formula, it is possible to
make the size of sulfide inclusions in steel small. This leads to
an increase in the number of sulfide inclusions containing less
than 1.0 mass % Ca, whereby good chip disposability can be
obtained. For further stably obtaining good chip disposability,
n.sub.1/n.sub.0 is desirably not more than 0.08. The ni is
preferably as small as possible and may be equal to 0 (zero).
[0046] The number n.sub.2, per mm.sup.2, of those oxide inclusions
in which the total content of CaO, Al.sub.2O.sub.3, SiO.sub.2 and
TiO.sub.2 is not less than 80 mass % and in which the contents of
these four oxides are within the ranges of CaO: 5-60%,
Al.sub.2O.sub.3: 5-60%, SiO.sub.2: 10-80% and TiO.sub.2: 0.1-40%,
with the sum of them being taken as 100% by mass, among the oxide
inclusions should satisfy the following relation:
n.sub.2.gtoreq.10 (3)
[0047] The number of the oxide inclusions defined above should be
not less than 10 because when it is less than 10, hard oxides
having a composition of high melting point, such as
Al.sub.2O.sub.3, are formed in addition to oxides having a
composition of low melting point, which are formed by addition of
Ca and Ti, and accordingly, no tool life prolonging effect can be
obtained.
[0048] The reason why the ranges of the contents of the oxides in
the inclusions are respectively restricted is that the oxides have
a low melting point in these composition ranges. Within this
restricted composition range, these oxides become soft with the
increasing temperature during cutting and, therefore, the oxides
will not promote the wear of the tool but contribute to the
prolongation of the tool life. Outside this composition range, the
melting points of the oxides rise and the hardness thereof
increases, and the oxides thus promote the wear of the tool, hence
the tool life is shortened.
[0049] For causing the inclusions in steel to be in such a state or
form as mentioned above and for attaining those mechanical
characteristics and machinability required of steels for machine
structural use, the contents of the components to be contained in
the steel must be restricted as mentioned below.
[0050] The content of C should be 0.1-0.6%. C is an important
element governing the properties relating to the strength of
steels, and the content thereof is generally selected taking the
mechanical properties into consideration. When the C content is
below 0.1%, the mechanical properties required of crankshafts and
other automotive mechanical parts cannot be obtained. On the other
hand, when it exceeds 0.6%, the tool life is markedly shortened and
the desired machinability can hardly be obtained. For obtaining
those mechanical properties, hardness and toughness, fatigue
strength and machinability which are required of crankshafts and
other automotive mechanical parts, it is desirable that the C
content be 0.30-0.55%.
[0051] The content of Si should be 0.01-2.0%. Si is an element
essential for attaining the oxide composition according to the
invention, and it is contained also for the purpose of deoxidizing
molten steel. At a content below 0.01%, the desired oxide
composition cannot be obtained. At levels exceeding 2.0%, its
effects saturate and, furthermore, a decrease in toughness of steel
is caused. Therefore, the Si content should be 0.01-2.0%. A more
preferred Si content range for stably obtaining the desired oxide
composition, without deteriorating the mechanical characteristics,
is 0.15-1.0%.
[0052] The content of Mn should be 0.2-2.0%. Mn is an important
element for forming sulfide inclusions greatly effective in
improving the machinability. It has a molten steel deoxidizing
effect as well. In addition, when S is caused to be contained for
improving the machinability, Mn is effective in preventing hot
workability of steel materials from deteriorating and, for
producing this effect, a content thereof not less than 0.2% is
essential. At levels exceeding 2.0%, however, resistance to cutting
increases. Thus, the Mn content should be 0.2-2.0%. In the case of
steels to be used after heat treatment, Mn is an element greatly
contributing to the hardenability, and the content for that purpose
is appropriately selected within the above range. On that occasion,
a more preferred Mn content range is 0.4-1.70%.
[0053] The content of S should be 0.005-0.2%. S is necessary for
improving the machinability. It binds with Mn etc. and forms
sulfide inclusions. The sulfide inclusion, MnS, readily changes its
shape in the process of steel solidification due to the addition of
Ca and Ti and, therefore, the shape of the MnS type sulfide
inclusions is specified simultaneously according to the present
invention. At a content below 0.005%, no machinability improving
effect is obtained and, at an excessively high content, the hot
workability and toughness of steel deteriorate. Therefore, its
content should be within the range of 0.005-0.2%. Within this
range, good machinability and mechanical properties can be
obtained. For attaining both appropriate mechanical characteristics
and good machinability of steels for machine structural use after
heat treatment, for instance, the S content is desirably
0.03-0.12%.
[0054] The content of Al (sol. Al, namely acid-soluble Al) should
be not more than 0.009%. Al has a great deoxidizing effects of
molten steel and is added for adjusting the level of deoxidation.
However, Al.sub.2O.sub.3, which is formed as a result of
deoxidation, is hard and shortens the tool life and, therefore, the
upper limit for Al is set at 0.009% for avoiding an increase of
Al.sub.2O.sub.3 content.
[0055] Within this range, the frequency of formation of
Al.sub.2O.sub.3 itself and oxides whose main component is
Al.sub.2O.sub.3 can be reduced. A small amount of Al as a
deoxidizer, which is used for rapid reduction in oxygen content at
the initial stage of steelmaking, or Al inevitably coming from raw
material such as ferro alloy will not cause any problem since such
Al is mostly used for the formation of CaO--Al.sub.2O.sub.3--S-
iO.sub.2--TiO.sub.2 oxides. Therefore, the Al content should be not
more than 0.009%, without necessity for setting any particular
lower limit. For more stable formation of the above mentioned
oxide, the Al content is desirably not more than 0.005%.
[0056] The content of Ti should be not less than 0.001% but less
than 0.04%. Ti is effective in stably forming oxides comprising
CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.2 and making them finer
and, therefore, is an element essential in the steel of the
invention. While those low-melting point oxides favorably
influencing the machinability can also be formed in
CaO--Al.sub.2O.sub.3--SiO.sub.2 system without Ti, the effects are
enhanced when TiO.sub.2 is contained in the oxide. When the Ti
content is below 0.001%, those effects will not be produced. At
levels of 0.04% or more, not only do the effects reach a point of
saturation but also the precipitation of hard TiN increases,
reducing the tool life. A more preferred Ti content for stably
forming oxides favorable for the machinability is within the range
of 0.005-0.025%.
[0057] The Ca content should be 0.0001-0.01%. Ca is effective in
improving the tool life and is necessary for the formation of
oxides comprising CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.2, which
are effective in improving the machinability. Below 0.0001%, such
effects are not produced to a satisfactory extent. On the other
hand, at levels exceeding 0.01%, the above-mentioned oxide can no
more be formed and, in addition, the cost of production increases
since the efficiency of addition of Ca is low. In addition, the
amount of MnS containing Ca as solid solution increases and MnS
becomes coarse. Thus, the number of MnS inclusions decreases and
the desired chip disposability improving and other effects cannot
be obtained. A more preferred Ca content for more stably attaining
the condition of inclusions as defined in accordance with the
invention is within the range of 0.0005-0.005%. For attaining the
condition or form of inclusions which is defined herein and suited
for machinability improvement, the steelmaking conditions before
and after addition of Ca should be taken into consideration.
[0058] The content of O (oxygen) should be 0.001-0.01%. Oxygen is
an important element for the formation of
CaO--Al.sub.2O.sub.3--SiO.sub.2--T- iO.sub.2 oxides favorable for
machinability improvement and for attaining the form and number of
sulfide inclusions, which are favorable for machinability
improvement. At a level below 0.001%, such effects are not
sufficient but it becomes rather difficult to obtain oxide
inclusions in those forms favorable for machinability improvement.
On the other hand, at content levels exceeding 0.01%, sulfide
inclusions, including MnS and so forth, become coarse and, in
addition, the amount of oxide inclusions increases, leading to
deterioration of not only machinability but also steel material
characteristics, such as a decrease in toughness. For more
certainly and stably obtaining those forms of inclusions that are
defined herein, the oxygen content is desirably not more than
0.005%.
[0059] The forms of sulfide inclusions and the composition of oxide
inclusions, which serve to improve the chip disposability and
prolong the tool life, are controlled in the process of
steelmaking, so that it is important to control of the steelmaking
process.
[0060] An example of the melting procedure for obtaining the forms
and composition of inclusions as defined herein is given and
explained below. However, the method of producing the steels
according to the invention should not be limited to the method of
production described hereinafter.
[0061] First, molten steel containing a small amount of carbon is
subjected, in a state of a low Al content, to vacuum treatment, for
instance, for adjusting the excess oxygen content. Then, the
contents of the main elements C, Si, Mn and S and other elements
are adjusted to the respective intended levels and then the
dissolved oxygen content is preliminarily adjusted. On that
occasion, Al may be added for adjusting the dissolved oxygen
content, if necessary. The Al content resulting from this addition
should be not more than 0.009%, preferably not more than 0.005%, as
mentioned hereinabove. Thereafter, Ti is added and the molten steel
is finally treated with Ca, followed by casting to give ingots or
blooms.
[0062] The reason why the production method according to the above
procedure is desirable is as mentioned below.
[0063] When the excess oxygen is removed from the molten steel
containing a small amount of carbon, the oxides formed by
deoxidation by Mn and Si added on the occasion of adjusting the
contents of the main components become the oxides not containing
excess Al.sub.2O.sub.3 as compared with the case where Al is added.
In composition adjustment, it is necessary to make adjustments so
that the amount of dissolved oxygen may not become too low due to
deoxidizing reactions of C, Mn and Si. The purpose of the dissolved
oxygen level adjustment is to cause Ca added before casting to form
the oxide to thereby prevent the formation of Ca-based sulfides
capable of causing the formation of MnS containing Ca as a solute.
Then, in some cases, Al is added for adjusting the dissolved oxygen
content, if necessary. However, excess amounts of Al.sub.2O.sub.3
type oxides are not formed because the Al addition is made in a
minimum necessary amount after the oxygen concentration and the
composition of oxide inclusions have already been adjusted.
Nevertheless, the presence of Al.sub.2O.sub.3 causes a reduction in
tool life, so that the Al content at that stage is required to be
not more than 0.009%, more desirably not more than 0.005%.
[0064] Then, Ti is added, whereupon deoxidation further proceeds.
The formed oxide of Ti combines with already existing oxides to
give thermodynamically more stable forms, which are effective in
preventing the formation of large inclusions and uniformly
dispersing inclusions about 1 to 10 .mu.m in size. The subsequent
treatment with Ca is made by adding calcium-silicon or ferro alloy.
Ca is hardly soluble in molten steel and reacts with oxygen in the
molten steel and with oxides dispersed therein, whereby
CaO--Al.sub.2O.sub.3--SiO.sub.2--TiO.sub.2 oxides are formed.
[0065] In accordance with one aspect of the invention, the steel
for machine structural use comprises the above-mentioned
components, with the balance being Fe and impurities. The following
upper limits are set to the contents of P and N among the
impurities.
[0066] P: not more than 0.1%
[0067] P is an element appearing in steel as an impurity. It has a
solid solution strengthening effect and a hardenability improving
effect. However, it deteriorates the toughness of steel, so that
the range of not more than 0.1% is selected as a range in which the
adverse effect is not so significant. Its content is desirably not
more than 0.05%, and the less, the better.
[0068] N: not more than 0.02%
[0069] N, when it coexists with Al, forms fine nitride, effectively
making steel crystal finer. However, in accordance with the
invention, the Al content in steel is restricted to a low level,
hence such effects cannot be expected. Rather, N binds to the
above-mentioned Ti to form TiN, which may possibly deteriorate the
tool life. Therefore, it is desirable that its content be as low as
possible. At levels not more than 0.02%, the adverse effects are
produced only to a slight extent. Hence, the allowable upper limit
is set at 0.02%. A more preferred range is not more than
0.015%.
[0070] In accordance with another aspect of the invention, the
steel for machine structural use comprises, in addition to the
components mentioned above, one or more components selected from
the first group and/or the second group given below.
[0071] First group: Cr: 0.02-2.5%, V: 0.05-0.5%, Mo: 0.05-1.0%, Nb:
0.005-0.1%, Cu: 0.02-1.0% and Ni: 0.05-2.0%;
[0072] Second group: Se: 0.0005-0.01%, Te: 0.0005-0.01%, Bi:
0.05-0.3% and rare earth elements: 0.0001-0.0020%.
[0073] The components belong to the above first group all
contribute to improvements in strength of steels. The components
belonging to the second group contribute to improvements in
machinability of steels. The contents of these elements are
restricted for the following reasons.
[0074] Cr: 0.02-2.5%
[0075] Cr is effective in improving the hardenability of steels and
is preferably added in alloy steels for machine structural use. A
content of not less than 0.02% is preferred for the purpose of
improving the hardenability but, at levels exceeding 2.5%, the
hardenability becomes excessively high, lowering the endurance
ratio and yield ratio and further deteriorating the machinability.
Therefore, the Cr content should be 0.02-2.5%.
[0076] Mo: 0.05-1.0%
[0077] Mo is effective in making the ferrite-pearlite structure
finer and, when heat refining is carried out, it is effective in
improving the hardenability and toughness. For securing such
effects, its content is desirably not less than 0.05%. However, at
levels exceeding 1.0%, the effects reach a point of saturation and
the fatigue strength may rather be reduced. Further, the cost
increases. Therefore, the Mo content should be 0.05-1.0%.
[0078] Ni: 0.05-2.0%
[0079] Ni is effective in improving the strength of steels through
solid solution strengthening and also in improving the
hardenability and/or toughness. For securing such effects, it is
desirable that its content be not less than 0.05%. At a level
exceeding 2.0%, however, the above effects reach a point of
saturation and, in addition, the hot workability deteriorates.
Therefore, an appropriate content of Ni is 0.05-2.0%.
[0080] Cu: 0.02-1.0%
[0081] Cu is effective in improving the hardenability of steels.
When such effect is desired, it is recommended that Cu be contained
at a level of not less than 0.02%. Furthermore, it is effective in
improving the strength of steels through precipitation
strengthening and, for producing this effect, its content of not
less than 0.1% is desirable. However, at a content level exceeding
1.0%, deterioration in hot workability may be induced or the
Cu-containing precipitates become coarse, whereby the above effects
are lost. Therefore, the Cu content should be 0.02-1.0%.
[0082] V: 0.05-0.5%, Nb: 0.005-0.1%
[0083] V and Nb precipitate as fine nitrides or carbonitrides and
thus improve the strength of steels. For securing this effect, it
is desirable that the V content be not less than 0.05% and the Nb
content not less than 0.005%. However, at V content exceeding 0.5%
or Nb content exceeding 0.1%, not only the above effect reaches a
point of saturation but also the nitrides and carbides are formed
in excessive amounts, whereby the machinability of steels
deteriorates and the toughness also decreases. Therefore, the V
content should be 0.05-0.5% and the Nb content 0.005-0.1%.
[0084] Se: 0.0005-0.01%, Te: 0.0005-0.01%
[0085] Se and Te react with Mn to form MnSe and MnTe, respectively,
and improve the machinability of steels. For producing this effect,
the contents of Se and Te are each desirably not less than 0.0005%.
At content levels of Se and Te exceeding 0.01%, however, that
effect reaches a point of saturation and the hot workability is
rather deteriorated. Therefore, an appropriate Se content and an
appropriate Te content are 0.0005-0.01% respectively.
[0086] Bi: 0.05-0.3%
[0087] Bi improves the machinability of steels. This is presumably
due to its formation of low-melting point inclusions and its
lubricating effect in the step of machining, like Pb. For securing
that effect, its content is recommendably not less than 0.05%.
However, when it exceeds 0.3%, not only the effect reaches a point
of saturation but also the hot workability is worsened. Therefore,
an appropriate content of Bi is within the range of 0.05-0.3%.
[0088] Rare earth elements: 0.0001-0.0020%
[0089] When rare earth elements are contained in steels, they form
inclusions including sulfides and increase the number of sulfide
inclusions, so that an machinability improving effect is obtained.
The rare earth elements such as La, Ce and Nd, and others are
called "REM". Mischmetal may also be used for adding rare earth
elements. When one or more of rare earth elements is added at a
level of not less than 0.0001%, the above effect is produced. For
obtaining the effect with more certainty, they are desirably added
at a level of not less than 0.0005%. At a level above 0.0020%,
however, the proportion of oxides and/or sulfides containing rare
earth elements increases; accordingly, the desired inclusion form
cannot be obtained, hence the machinability cannot be improved.
Therefore, an appropriate content of rare earth elements is within
the range of 0.0001-0.0020%.
EXAMPLE
[0090] Steels having the respective chemical compositions shown in
Table 1 and Table 2 were melted and cast to give 150 kg ingots.
Some steels shown in Table 2 were melted by the procedure to be
mentioned later herein. In Table 2, the steels Nos. 74 and 75 are
Pb-containing steels.
[0091] (1) Each molten steel, in a state containing a small amount
of carbon, was subjected to vacuum treatment for excess oxygen
adjustment in a low Al content state.
[0092] (2) Then, the furnace inside was adjusted to an argon
atmosphere and, thereafter, the main components C, Si, Mn and S and
other elements were adjusted to the desired levels and, at the same
time, iron oxide was added, if necessary, to adjust the dissolved
oxygen content. Then, Al was added, if necessary, for further
adjustment of the dissolved oxygen content.
[0093] (3) Thereafter, Ti was added and, after the final treatment
with Ca, the melt was cast to give ingots or blooms.
[0094] The steels shown in Table 1 are steels falling within the
composition range defined in accordance with the present invention.
The steels shown in Table 2 are steels failing to fall within that
composition range.
[0095] Among the steels shown in Table 2, those steels differing in
the form of inclusions from the steels of the invention were melted
in the following manner, even when they were within the same
composition range. Thus, in melting those having a high oxygen
content; the vacuum treatment in a state containing a small amount
of carbon was omitted or iron oxide was added in excess for
adjusting the dissolved oxygen content in the intermediate stage.
In melting those having a high Al concentration, Al was added at
the stage of adjusting the main components. In cases where further
sufficient deoxidation was carried out, Al was added for
deoxidation immediately before the addition of Ca, which was
performed in the conventional manner, according to the chemical
analysis and the like. For those steels for which no further
deoxidation with Al was conducted, the dissolved oxygen level
adjustment by addition of iron oxide or the like was not carried
out after the deoxidation with C, Si and Mn, but Ti and Ca were
added immediately before casting.
[0096] In this process of melting, the excess Ca, which does not
contribute to the deoxidation reaction, forms CaS in the molten
steel stage because of its high affinity for S and the CaS serves
as nuclei for the formation of MnS which crystallizes out
subsequently. As a result, in cases where the excess Ca, whish does
not contribute to the deoxidation reaction, is contained in a state
after sufficient deoxidation, it forms CaS in the molten steel and
MnS crystallizes out utilizing the CaS as nuclei for the formation
of MnS. Therefore, the number (n.sub.1) of MnS inclusions
containing not less than 1% of Ca as a solute increases and the
left term "n.sub.1/n.sub.0" of the formula (2) exceeds 0.1. As a
result, sulfide coarsening is caused and the total number (n.sub.0)
of inclusions decreases. Therefore, the relation (1), namely
"n.sub.0/S (%).gtoreq.2500" is not satisfied, hence the desired
chip disposability cannot be obtained.
[0097] Each steel ingot was heated at 1250.degree. C. and then
hot-forged at temperatures up to 1000.degree. C. to give a round
bar with a diameter of 70 mm and, after forging, the bar was
air-cooled to room temperature. Test specimens were taken from the
thus-obtained round bar at a site of 17.5 mm deep from the bar
surface, namely at a site half the radius of the round bar, the
cross section of each specimen parallel to the direction of working
was mirror-polished and observed at a magnification of 400 using an
EPMA in not less than 20 fields of view per specimen, and those
sulfide and oxide inclusions not less than 1 .mu.m in circle
equivalent diameter (diameter of a circle equal in area to the
grain) were counted. Then, not less than ten sulfide and oxide
inclusions randomly selected for each field of view were
quantitatively analyzed and the compositions thereof were
determined.
1 TABLE 1 Chemical Composition (mass %, Fe:bal.) No C Si Mn P S Ti
sol. Al Ca O N Steel of This Invention 1 0.39 0.26 0.52 0.008 0.045
0.018 0.005 0.0023 0.0031 0.0057 2 0.38 0.21 0.55 0.016 0.048 0.020
0.002 0.0021 0.0031 0.0084 3 0.38 0.20 0.57 0.018 0.050 0.007 0.003
0.0013 0.0024 0.0144 4 0.41 0.21 1.25 0.012 0.053 0.018 0.003
0.0017 0.0021 0.0110 5 0.50 0.20 0.79 0.013 0.056 0.003 0.002
0.0025 0.0029 0.0123 6 0.40 0.22 0.59 0.025 0.058 0.020 0.003
0.0011 0.0017 0.0102 7 0.36 0.25 0.79 0.021 0.068 0.035 0.002
0.0014 0.0038 0.0076 8 0.47 0.19 1.15 0.025 0.074 0.021 <0.002
0.0017 0.0025 0.0102 9 0.43 0.19 1.52 0.022 0.075 0.035 0.002
0.0024 0.0035 0.0094 10 0.41 0.17 1.21 0.016 0.106 0.007 0.007
0.0018 0.0032 0.0144 11 0.44 0.17 1.26 0.016 0.108 0.008 0.002
0.0030 0.0030 0.0120 12 0.55 0.25 1.24 0.020 0.160 0.006 0.003
0.0015 0.0024 0.0095 13 0.53 0.21 0.74 0.013 0.058 0.019 0.002
0.0025 0.0029 0.0111 Cr:0.10 14 0.38 0.53 1.45 0.015 0.061 0.007
0.001 0.0018 0.0029 0.0135 Cr:0.14 15 0.39 0.55 1.50 0.016 0.065
0.006 0.002 0.0012 0.0023 0.0141 Cr:0.12 16 0.52 0.17 0.74 0.022
0.069 0.004 <0.002 0.0016 0.0038 0.0124 Cr:0.09 17 0.35 0.17
1.28 0.018 0.092 0.006 0.002 0.0012 0.0033 0.0130 Cr:0.20 18 0.41
0.20 1.29 0.018 0.093 0.002 0.002 0.0034 0.0036 0.0145 Cu:0.02 19
0.47 0.25 1.40 0.020 0.068 0.008 0.001 0.0019 0.0027 0.0124 Ni:0.10
20 0.42 0.24 1.23 0.029 0.065 0.020 0.004 0.0019 0.0028 0.0108
Nb:0.05 21 0.42 0.22 1.26 0.010 0.098 0.005 0.002 0.0029 0.0035
0.0082 Nb:0.10 22 0.41 0.20 1.18 0.022 0.072 0.021 0.004 0.0013
0.0020 0.0110 Mo:0.10 23 0.37 0.25 1.29 0.013 0.094 0.032 0.002
0.0024 0.0026 0.0087 Mo:0.10 24 0.45 0.18 1.16 0.021 0.156 0.017
0.002 0.0020 0.0029 0.0107 V:0.10 25 0.41 0.46 0.75 0.016 0.036
0.010 0.002 0.0016 0.0029 0.0148 Cr:0.10, V:0.07 26 0.48 0.27 1.42
0.019 0.065 0.007 0.002 0.0017 0.0025 0.0105 Cr:0.10, V:0.10 27
0.40 0.18 1.24 0.014 0.110 0.011 <0.002 0.0011 0.0029 0.0128
Cr:0.25, V:0.10 28 0.39 0.17 1.22 0.014 0.106 0.024 <0.002
0.0030 0.0041 0.0119 Ni:0.14, Cr:0.25, V:0.09 29 0.43 0.20 1.17
0.017 0.042 0.026 0.003 0.0020 0.0025 0.0080 Se:0.0025 30 0.38 0.20
1.20 0.018 0.055 0.018 0.003 0.0015 0.0025 0.0078 Te:0.0020 31 0.40
0.22 1.24 0.023 0.088 0.030 <0.002 0.0020 0.0030 0.0087
REM:0.0009 32 0.43 0.21 1.30 0.020 0.090 0.004 0.003 0.0023 0.0027
0.0079 Se:0.005 33 0.40 0.22 1.17 0.011 0.155 0.020 0.002 0.0013
0.0018 0.0085 Bi:0.07 34 0.46 0.20 1.23 0.021 0.040 0.024 0.003
0.0016 0.0022 0.0081 Cr:0.08, Bi:0.05 35 0.42 0.18 1.15 0.024 0.052
0.023 0.003 0.0021 0.0034 0.0116 V:0.09, Bi:0.07 36 0.45 0.19 1.24
0.026 0.062 0.020 <0.002 0.0014 0.0019 0.0079 Cr:0.10, Te:0.0035
37 0.45 0.22 1.21 0.024 0.063 0.030 0.003 0.0011 0.0014 0.0076
V:0.10, Te:0.0020, REM:0.0009 38 0.39 0.22 1.16 0.015 0.065 0.015
0.002 0.0013 0.0019 0.0075 Cr:0.10, REM:0.0010 39 0.46 0.17 1.16
0.028 0.110 0.019 0.004 0.0017 0.0028 0.0096 Cu:0.04, Bi:0.08 40
0.37 0.21 1.19 0.013 0.134 0.034 0.003 0.0015 0.0020 0.0104
Bi:0.09, REM:0.0011
[0098]
2 TABLE 2 Chemical Composition (mass %, Fe:bal.) No C Si Mn P S Ti
sol. Al Ca O N Comparative Example 41 0.50 0.19 0.81 0.015 0.053
0.007 0.003 0.0020 0.0015 0.0105 42 0.53 0.20 0.76 0.014 0.058
0.003 0.002 0.0033 0.0041 0.0116 43 0.48 0.22 1.25 0.010 0.055
0.008 0.002 0.0020 0.0055 0.0115 44 0.45 0.21 1.34 0.020 0.098
0.015 0.003 0.0031 0.0010 0.0111 45 0.39 0.24 1.26 0.020 0.099
0.010 0.005 0.0018 0.0019 0.0107 46 0.42 0.21 1.20 0.018 0.095
0.009 0.003 0.0022 0.0049 0.0112 47 0.43 0.22 1.26 0.019 0.094
0.008 0.002 0.0018 0.0054 0.0123 48 0.44 0.19 1.25 0.014 0.102
0.008 0.002 0.0027 0.0025 0.0125 49 0.42 0.18 1.21 0.018 0.109
0.005 0.004 0.0025 0.0021 0.0110 50 0.36 0.60 1.46 0.015 0.172
0.010 <0.002 0.0030 0.0021 0.0160 51 0.39 0.65 1.44 0.015 0.175
0.011 <0.002 0.0030 0.0031 0.0170 52 0.40 0.30 1.10 0.018 0.095
0.020 0.008 0.0035 0.0025 0.0106 V:0.10 53 0.42 0.22 1.20 0.016
0.097 0.009 0.002 0.0025 0.0020 0.0098 Cr:0.10 54 0.45 0.20 1.19
0.016 0.100 0.015 0.007 0.0030 0.0025 0.0111 Mo:0.10 55 0.46 0.25
1.14 0.013 0.103 0.007 0.002 0.0021 0.0020 0.0104 Cu:0.03 56 0.38
0.16 1.24 0.019 0.105 0.015 0.004 0.0026 0.0018 0.0100 Te:0.0014 57
0.36 0.27 1.19 0.024 0.107 0.018 0.003 0.0024 0.0018 0.0105
Mg:0.0010 58 0.45 0.21 1.16 0.018 0.112 0.020 0.006 0.0013 0.0010
0.0099 REM:0.0008 59 0.38 0.64 1.38 0.015 0.180 0.012 <0.002
0.0030 0.0024 0.0135 Cr:0.18, V:0.14 60 0.38 0.18 1.22 0.014 0.099
0.008 <0.002 0.0033 0.0021 0.0108 Ni:0.15, Cr:0.25, V:0.09 61
0.37 0.20 1.20 0.020 0.172 0.006 <0.002 0.0025 0.0017 0.0133
Cu:0.11, Ni:0.06, Cr:0.19, Mo:0.03, V:0.12 62 0.45 0.005* 1.15
0.018 0.108 0.007 <0.002 0.0021 0.0095 0.0108 63 0.51 0.17 0.81
0.013 0.002* 0.008 0.004 0.0030 0.0040 0.0080 64 0.40 0.19 2.10*
0.016 0.101 0.026 <0.002 0.0035 0.0053 0.0115 65 0.85* 0.18 1.15
0.015 0.099 0.018 <0.002 0.0029 0.0038 0.0125 66 0.52 0.19 0.92
0.019 0.054 0.032 0.020* 0.0027 0.0010 0.0134 67 0.50 0.24 1.26
0.017 0.106 0.025 0.010* 0.0003 0.0019 0.0111 68 0.38 0.22 1.22
0.012 0.180 0.002 <0.002 <0.0001* 0.0016 0.0094 69 0.40 0.21
1.19 0.025 0.104 0.009 0.035* 0.0020 0.0025 0.0101 70 0.42 0.19
1.26 0.021 0.095 0.011 0.030* 0.0012 0.0019 0.0115 71 0.46 0.18
1.22 0.017 0.097 0.013 0.020* 0.0017 0.0020 0.0107 72 0.45 0.20
1.25 0.020 0.098 0.005 0.040* 0.0014 0.0020 0.0105 73 0.48 0.13
1.24 0.015 0.116 <0.001* 0.003 0.0029 0.0025 0.0127 74 0.50 0.25
1.20 0.020 0.060 0.001 0.002 0.0018 0.0024 0.0090 Pb:0.13* 75 0.46
0.45 1.00 0.020 0.068 0.004 <0.002 0.0015 0.0025 0.0085
Pb:0.15*
[0099] Based on the thus-found total number (n.sub.0) of sulfide
inclusions per unit specimen area (1 mm.sup.2) and the result of
analysis for S, "n.sub.0/S (%)" was calculated. Then, the number of
those sulfide inclusions containing not less than 1.0 mass % of Ca
was determined, and "n.sub.1/n.sub.0" was calculated.
[0100] For the oxide inclusions analyzed in the above manner, the
number (n.sub.2) of those oxide inclusions in which the sum of the
constituents CaO, Al.sub.2O.sub.3, SiO.sub.2 and TiO.sub.2
accounted for not less than 80% by mass, with CaO: 5-60%,
Al.sub.2O.sub.3: 5-60%, SiO.sub.2: 10-80% and TiO.sub.2: 0.1-40%
when the sum of CaO, Al.sub.2O.sub.3, SiO.sub.2 and TiO.sub.2 was
taken as 100% by mass was determined. The results of these
examinations as to inclusions are summarized in Table 3 and Table
4. Mark "*" in Table 4 indicates values not satisfying the
conditions of this invention or not reaching aimed properties.
3TABLE 3 Tool Life n.sub.0 n.sub.1 n.sub.2 Chip Disposability
(Number of No (number/mm.sup.2) n.sub.0/S(%) (number/mm.sup.2)
n.sub.1/n.sub.0 (number/mm.sup.2) Index f Drillings) Steel of This
Invention 1 258 5733 13 0.050 18 1156 81 2 244 5072 11 0.045 17
1226 82 3 245 4900 7 0.029 17 1220 98 4 287 5398 10 0.035 12 1166
85 5 321 5732 13 0.040 26 1214 125 6 254 4379 10 0.039 15 1155 92 7
365 5368 31 0.085 24 941 125 8 378 5121 16 0.042 14 989 87 9 349
4629 24 0.069 27 889 97 10 460 4355 9 0.019 12 1119 107 11 472 4362
18 0.038 20 944 116 12 536 3350 8 0.015 24 894 155 13 319 5502 8
0.025 35 1086 137 14 229 3754 14 0.061 15 934 89 15 284 4369 13
0.046 12 1015 101 16 283 4099 15 0.053 16 971 116 17 509 5542 4
0.008 24 1039 116 18 532 5720 12 0.023 19 1183 132 19 255 3750 11
0.043 14 1000 94 20 310 4793 13 0.042 13 928 94 21 582 5958 13
0.023 19 1106 115 22 335 4629 10 0.030 18 1119 99 23 558 5947 19
0.034 16 1186 124 24 571 3660 18 0.032 21 731 127 25 219 6083 16
0.073 19 1500 84 26 320 4923 15 0.047 15 1169 85 27 423 3850 15
0.035 22 927 125 28 604 5697 8 0.013 14 858 129 29 259 6128 20
0.077 18 1538 89 30 262 4787 7 0.027 21 1023 102 31 465 5288 12
0.026 25 955 107 32 573 6340 11 0.019 32 1153 116 33 529 3413 6
0.011 19 748 125 34 235 5826 7 0.030 24 1463 80 35 268 5123 8 0.030
24 1434 98 36 310 5005 11 0.035 14 1114 89 37 369 5838 16 0.043 19
1187 86 38 317 4855 15 0.047 22 1057 85 39 496 4509 10 0.020 19 791
121 40 528 3940 16 0.030 22 799 125
[0101]
4TABLE 4 Tool Life n.sub.0 n.sub.1 n.sub.2 Chip Disposability
(Number of No (number/mm.sup.2) n.sub.0/S(%) (number/mm.sup.2)
n.sub.1/n.sub.0 (number/mm.sup.2) Index f Drillings) Comparative
Example 41 116 2189* 18 0.155* 13 604* 104 42 121 2086* 20 0.165*
24 586* 109 43 135 2455* 14 0.104* 25 673* 98 44 142 1449* 17
0.119* 32 480* 139 45 145 1465* 16 0.110* 20 626* 135 46 225 2368*
23 0.102* 18 674* 119 47 218 2319* 22 0.101* 16 638* 105 48 245
2402* 25 0.102* 12 686* 128 49 145 1330* 15 0.103* 15 413* 126 50
278 1616* 29 0.104* 15 517* 157 51 304 1737* 31 0.102* 19 497* 154
52 187 1968* 20 0.107* 10 695* 127 53 169 1742* 18 0.107* 16 670*
121 54 184 1840* 22 0.120* 11 610* 108 55 194 1883* 20 0.103* 11
680* 117 56 179 1705* 18 0.101* 14 476* 124 57 244 2465* 32 0.131*
36 595* 126 58 198 1768* 20 0.101* 18 571* 119 59 326 1811* 33
0.101* 24 411* 158 60 244 2465* 68 0.277* 36 596* 126 61 312 1814*
33 0.106* 34 558* 165 62 210 1944* 25 0.119* 10 454* 118 63 73
36500 4 0.055 11 22500 8* 64 477 4719 23 0.048 19 977 34* 65 490
4945 32 0.065 16 906 7* 66 346 6407 12 0.035 4* 1463 20* 67 452
4264 0 0.000 0* 1104 62* 68 555 3083 0 0.000 0* 711 82* 69 421 4048
15 0.036 0* 904 63* 70 457 4811 12 0.026 0* 1032 50* 71 448 4619 18
0.040 2* 1021 55* 72 498 5082 17 0.034 1* 1092 45* 73 478 4121 29
0.061 6* 629 78* 74 145 2417 12 0.083 14 1150 101 75 123 2236* 8
0.065 3* 1044 105
[0102] The machinability evaluation was carried out in the
following manner. Cylindrical test specimens with a length of 60 mm
were taken from the round bar with a diameter of 70 mm as prepared
in the manner mentioned above, and the cross section of each
specimen was subjected to a drilling test in the perpendicular
direction. As for the drilling conditions, a straight shank drill
made of a high-speed steel and having a diameter of 6 mm was used,
together with a water-soluble cutting fluid (emulsion type), and
the feed rate was 0.15 mm/rev, the number of revolutions was 980
rpm, and the hole depth was 50 mm.
[0103] In this test, the tool life was evaluated in terms of the
number of drillings after which drilling was no more possible due
to the wear of the tip. The chip disposability was evaluated in
terms of the chip disposability index (f) as calculated by dividing
the number of chips cut out per unit mass as counted in the above
test by the S content (% by mass) of the relevant steel. It is
known that the number of chips per unit mass increases as the S
content in steel increases. When the S content is the same, the
chip disposability is better when the number of chips per unit mass
is greater. The results of these machinability evaluations are also
shown in Table 3 and Table 4.
[0104] As is seen from the numbers of inclusions and the
machinability evaluation results shown in Table 3 and Table 4, the
steels having a chemical composition within the range defined
herein and satisfying the conditions specified herein with respect
to the forms of sulfide and oxide inclusions, namely the steels
shown in Table 1, all gave better results with respect to the chip
disposability and tool life as compared with the steels shown in
Table 2, except the steels Nos. 74 and 75. It is evident that the
steels shown in Table 1 are comparable or superior in machinability
to the Pb-containing steels Nos. 74 and 75 given as reference
examples.
[0105] FIG. 1 is a graphic representation of the relationship
between chip disposability index and S content as drawn based on
the data shown in Table 3 and Table 4. The data for those steels
No. 63 to No. 73, which were particularly poor in tool life, have
been omitted. From this figure it is evident that the steels of the
invention are superior in chip disposability when the S content is
at the same level.
[0106] FIG. 2 is a graphic representation of the relationship
between chip disposability index and "n.sub.1/n.sub.0" as drawn
based on the data shown in Table 3 and Table 4. The data for those
steels Nos. 63-73, which were particularly poor in tool life, have
been omitted. From this figure, it is seen that the steels of the
invention which satisfy the condition "n.sub.1/n.sub.0.ltoreq.0.1"
are superior in chip disposability.
[0107] FIG. 3 is a graphic representation of the relationship
between chip disposability index and "n.sub.0/S (%)" as drawn based
on the data shown in Table 3 and Table 4. The data for those steels
Nos. 63-73, which were particularly poor in tool life, have been
omitted. From FIG. 3, it is seen that the steels of the invention
which satisfy the condition "n.sub.0/S (%).gtoreq.2500" are
superior in chip disposability.
[0108] FIG. 4 is a graphic representation of the relationship
between tool life and S content as drawn based on the data shown in
Table 3 and Table 4. The data for those steels Nos. 41-62, which
were particularly poor in chip disposability, have been omitted.
From this figure, it is seen that the steels of the invention are
superior in tool life when comparison is made on the same S content
level.
[0109] FIG. 5 is a graphic representation of the relationship
between tool life and n.sub.2 as drawn based on the data shown in
Table 3 and Table 4. In this figure, the data for those steels Nos.
41-62, which were particularly poor in chip disposability, have
been omitted. The data of the steels of the invention (steels Nos.
8-11, 17-18, 21, 23, 27-28, 31-32 and 39) having an S content
within the range of 0.074-0.119%, and the date of the comparative
steels Nos. 67 and 69-73 have been added for comparison at the same
S content level. From FIG. 5, it is evident that the steels of the
invention satisfying "n.sub.2.gtoreq.10" are superior in tool life
when comparison is made on the same S content level.
[0110] The steel for machine structural use according to the
invention is excellent in machinability, in particular chip
disposability, and in tool life prolonging effect as well, in spite
of containing no Pb. When this steel is used as a parts material
requiring machining, the production cost of the parts can be
markedly reduced.
* * * * *